Zirconia based ceramics have been used for many years, for instance as structural materials for orthopedic implants and prostheses as well as for dental implants and restorations. Such ceramics typically contain zirconia in its tetragonal form, which is metastable at room temperature and thus requires stabilizers such as Y2O3.
Ceramic materials based on partially stabilized zirconia, namely yttria stabilized tetragonal zirconia polycrystal (Y-TZP), are widely used and generally have favorable mechanical properties such as very high flexural strength. However, these ceramics exhibit a relatively modest fracture toughness (KIC) in the range of 4 to 5 MPa·m0.5. Moreover, Y-TZP materials have been found to be subject to the phenomenon of low temperature degradation (LTD), also referred to as aging, especially in the presence of water or a humid environment.
A different type of zirconia ceramics are the ceria stabilized zirconia ceramics (Ce-TZP). While these ceramics can show higher fracture toughness values than Y-TZP materials, they have only a moderate flexural strength, with conventional Ce-TZP ceramics generally exhibiting a flexural strength of not more than 700 MPa. This is insufficient for many medical and dental applications, which typically require a flexural strength of at least 800 MPa.
Yet another type of ceramics are the so-called ceramic matrix composites (CMC), which comprise at least two different crystalline phases. Typical examples are zirconia toughened alumina (ZTA) or alumina toughened zirconia (ATZ), but other matrices, such as SiC or Si3N4, are also known for this type of material.
U.S. Pat. No. 4,880,757, which is hereby incorporated by reference, discloses composite ceramic materials comprising a zirconia minor phase, which may be stabilized by Y2O3, and a spinel major phase.
Morita et al. (Scripta Materialia, 2005, 53, 1007-1012) describe a nanocrystalline composite comprising Y2O3-stabilized tetragonal zirconia and MgAl2O4 phases. However, the overall mechanical properties of this material are still not fully satisfactory.
U.S. Pat. No. 5,728,636, which is hereby incorporated by reference, describes a ceramic matrix composite consisting of a zirconia matrix stabilized with 8 to 12 mol % CeO2 and 0.05 to 4 mol % TiO2 and having Al2O3 as second component, which accounts for 0.5 to 50 vol % of the composite. EP 1 382 586 and EP 1 580 178, which are hereby incorporated by reference, describe similar composite materials, which comprise a first phase of zirconia stabilized with 10-12 mol % CeO2 and 20 to 60 or 70 vol % Al2O3 as a second phase.
One particular disadvantage of CeO2-stabilized zirconia based ceramic matrix composites of the prior art is that their intrinsic colors are unacceptable for dental restorative materials. Furthermore, due to their sensitivity to redox reactions, these materials exhibit pronounced color instability upon heat treatment at low oxygen partial pressure. For instance, when veneering such materials with glass ceramics, the application of vacuum within the firing chamber that is needed to achieve a homogenous densely sintered layering material results in a color change to give a highly unpleasant greenish appearance.
Furthermore, it has been found necessary to fully blast conventional Ce-TZP ceramic frameworks prior to veneering in order to achieve sufficiently high bond strength to the veneering. In comparison to the common instructions for use relating to 3Y-TZP dental materials, this procedure is totally different and might lead to confusion. Another disadvantage is the low wettability of the compositions used in the preparation of the prior art composites, which makes application of a glass-ceramic slurry for instance for veneering much more difficult. Thus, these materials are not suitable for many common dental applications, processes and materials.